DISCRIMINATION OF AGE OF GLACIAL DRIFT SHEETS 699 
and the apparent maturity is largely due to the fact that the pre- 
glacial topography was but slightly masked and not wholly buried as 
it was within the terminal moraines of the later sheets. The post- 
glacial drainage, with the exception of these slight divergences, has 
followed pre-glacial lines, and here the gradients of the drift-mantled 
slopes were sufficiently high to favor rapid erosion. 
East of Rock River the area covered by this same drift has a more 
youthful aspect, being a gently undulating upland plain with slight 
drumloidal ridges and occasional undrained sags. The borders of 
the tract are trenched by drainage lines but not greatly dissected. The 
amount of erosion of this upland, which extends southward to the 
valley of the Kishwaukee in southern Boone County, Illinois, appears 
but little greater than in similarly situated tracts within the Wisconsin 
terminal moraines, and, judging from these features alone, one would 
be very apt to conclude that the drift east of Rock River was consider- 
ably younger than that west of Sugar River, if not younger than that 
between the two streams. 
Passing to the upland prairie west of Rock River in Winnebago, 
Stephenson, Ogle, Lee, and Whiteside counties, Illinois, one finds 
extensive, undissected tracts, nearly flat or very gently sloping (Figs. 4 
and 5). - Only the borders of these tracts, where abrupt slopes drop 
down to the valleys, appear to be eroded. From this condition there 
is a gradual transition northward to the rather mature topography 
between Rock and Sugar rivers. We have thus distinct topographic 
differences in different parts of the area which affect the interpretation 
of the glacial history. 
On examining the degree of surficial alteration of this drift also 
notable differences appear. West of Rock River in Illinois, where 
there is such slight drainage dissection, the drift, beneath the thin 
coating of stoneless clayey loam or loess which is generally present, 
exhibits striking evidence of long exposure to leaching and oxidation. 
In many places but a scattering of pebbles lies between the loess 
or loam and the weathered surface of the limestone and these pebbles 
are all of the more insoluble kinds of rocks, mostly dense, fine-grained 
crystallines, cherts, quartzites, and vein quartz. Where there is a few 
feet of drift it is generally oxidized to a dark brownish color, is rather 
compact and sandy, not loose sand, but crumbling when dry into little 
